A relay has been standardized in the Institute of Electrical and Electronics Engineers (IEEE) 802.16 series for the purpose of removing a shadow area of a cell and for improving a system throughput. In addition, the introduction of the relay has also been discussed in the release 9 of 3rd Generation Partnership Project Long Term Evolution (3GPP LTE).
The 3GPP LTE classifies the relay into three types, i.e., a Layer (L1 relay, an L2 relay, and an L3 relay. With regard to the L1 relay, a relay evolved Node-B (eNB) is a repeater for transmitting the same signal as that of a macro eNB. With regard to the L2 relay, the relay eNB transmits its own broadcast information and control signaling information, and also performs scheduling. With regard to the L2 relay, a macro eNB that sets up a radio link to the relay eNB manages data communication with respect to the relay eNB. Because the relay eNB is regarded as a different entity from a User Equipment (UE), the radio link which is set up between the macro eNB and the relay eNB is handled specially in scheduling of the macro eNB. In general, the relay of the IEEE 802.16 series corresponds to the L2 relay. The L3 relay differs from the L2 relay in that the radio link which is set up between the macro eNB and the relay eNB and a radio link which is set up between the macro eNB and a typical UE are handled equally without distinction in the scheduling of the macro eNB. Additionally in the L3 relay, the macro eNB and the relay eNB create an X2 interface, and the X2 interface is also created with a radio link because the macro eNB and the relay eNB are connected only with the radio link.
Meanwhile, a process of performing a handover from a macro eNB to a relay eNB by a UE in the conventional an L3-relay system of 3GPP LTE is as follows. When the UE moves to a region close in distance to a service area of the relay eNB while receiving a service from the macro eNB, and thus a certain condition is satisfied, the macro eNB detects a handover of the UE on the basis of a measurement report message of the UE. Upon the detecting the handover of the UE, a serving eNB (i.e., the macro eNB) and a target eNB (i.e., the relay eNB) exchange a handover request message and a handover request response message through an X2 interface, and in this case, the relay eNB performs call admission control. Since the X2 interface is created by the macro eNB and the relay eNB through a relay link, the handover request message and the handover request response message also have to be transmitted through the radio link. If the radio link between the macro eNB and the relay eNB is not created (that is, if a connection is not established), the macro eNB and the relay eNB have to exchange a paging message, a Radio Resource Control (RRC) connection request message, an RRC connection setup message, and an RRC connection setup complete message to create the radio link. If the radio link between the macro eNB and the relay eNB is created, information on a radio link change which may occur between the macro eNB and the relay eNB due to a handover has to be sent by the macro eNB to the relay eNB, and this process is performed by exchanging an RRC connection reconfiguration message and an RRC connection reconfiguration complete message. As such, the handover of the UE from the macro eNB to the relay eNB requires exchange of not only an X2 signaling message but also an RRC signal message.
In the conventional technique, a handover process is separated from an RRC connection process and an RRC connection reconfiguration process, and the macro eNB does not perform call admission control even if it can perform the call admission control, which leads to the following problems. First, an amount of messages exchanged between the macro eNB and the relay eNB increases. If there is no connection established between the macro eNB and the relay eNB, the macro eNB has to exchange a paging message, an RRC connection request message, an RRC connection setup message, and an RRC connection setup complete message with the relay eNB before the macro eNB transmits a handover request message to the relay eNB, which causes a significantly long handover delay. Further, even if there is a connection established between the macro eNB and the relay eNB, a process of exchanging an RRC connection reconfiguration message and an RRC connection reconfiguration complete message between the macro eNB and the relay eNB is necessary. Second, when the relay eNB fails in call admission in a call admission control process, in many cases, it is because resources are not ensured over a radio link between the macro eNB and the relay eNB. The relay eNB performs call admission control for determining whether to admit a call request of the UE upon receiving the handover request message for requesting a call of the UE from the macro eNB. In this case, even if a large amount of radio link resources are present between the relay eNB and the UE, the relay eNB cannot admit the call request of the UE when an amount of radio link resources between the macro eNB and the relay eNB is limited.